1. Field of the Invention
This invention relates to the field of torque converters for an automatic transmission and the control of a bypass clutch that permits a variable speed difference between the driving and driven members of the torque converter. More particularly, the invention pertains to control of hydraulic pressure supplied to a torque converter bypass clutch, especially its response to an abrupt engine torque demand by a vehicle operator.
2. Description of the Prior Art
In an automatic transmission for a motor vehicle, the internal combustion engine is drivably connected to a torque converter having an impeller rotor fixed to the engine shaft and a turbine rotor driven hydrodynamically by the impeller. A typical torque converter also includes a bypass clutch, which, when engaged, mechanically connects the impeller and turbine thereby discounting the hydrodynamic connection between those components and, when disengaged, permits the conventional hydrodynamic drive between the turbine and impeller.
The torque converter bypass clutch may be controlled through operation of a solenoid-operated hydraulic valve, which pressurizes and vents the bypass clutch to engage and disengage the clutch in response to a signal applied to the solenoid. When the torque converter is opened, i.e., when the bypass clutch is disengaged, the hydrodynamic connection between the turbine and impeller absorbs and attenuates vibrations and other disturbances that are unacceptable in an automotive powertrain. However, because of the inherent losses associated with operation of a torque converter in the open mode, fuel economy is reduced. Torque converters can be controlled such that, throughout most of the range of operation, the bypass clutch is neither fully engaged nor fully disengaged but instead is modulated to produce a variable magnitude of slip, the difference between the speeds of the impeller and turbine. When operated in this way, the torque converter can be used to absorb vibrations, particularly when gear ratio changes are being made, by increasing the slip, thus permitting a greater portion of engine torque to be passed from the impeller to the turbine through hydrodynamic action. When chance of objectionable vibration and disturbance is absent, the torque converter can be more fully closed so that fuel economy is enhanced.
Generally, a control system for operating the bypass clutch responds to a vehicle operator's demand for a change in engine torque on the basis of changes made manually to the position of the throttle valve. In normal operation, these changes are made abruptly yet the system must respond quickly to the change in torque demand in order to satisfy the driver's expectations and satisfy fuel economy and powertrain vibration criteria. Delays are inherent in a powertrain of this type. Engine inertia causes a delay in producing a change in engine output in response to a throttle position change, and there are other delays in the control system associated with clutch inertia, hydraulic system inertia, viscosity, friction, and other such phenomena.
U.S. Pat. No. 4,757,886 describes a closed loop control system for controlling operation of a transmission bypass clutch on the basis of slip across the torque converter. The system employs measured slip as a feedback signal combined with a command signal to produce a slip error. Thereafter, a duty cycle correction on the basis of the difference between duty cycles in the current sampling period and the previous sampling period is developed and processed to produce a clutch pressure signal. That signal is used to vary the slip and is feed back to enhance stability of the duty cycle error signal.
U.S. Pat. No. 4,790,418 describes a technique for controlling the transfer of torque from an offgoing friction element to an oncoming friction element when a gear ratio change is made in an automatic transmission. Slip across the offgoing clutch is used to control engine speed. The time rate of change of torque in the offgoing clutch is used as a basis to change clutch pressure in accordance with a calculated torque slope.
U.S. Pat. No. 4,792,902 describes a system and a method for controlling ignition timing of an internal combustion engine in a driveline that includes a multiple speed ratio transmission. The spark timing of the engine is advanced and retarded on the basis of engine speed and transmission output speed by a control algorithm that improves the speed ratio control during gear ratio changes. None of these patents describes a feedforward technique for controlling the bypass clutch of an automatic transmission.